In U.S. Pat. No. 4,540,884 there is described a method of mass analyzing a sample by the use of a quadrupole ion trap. Basically, a wide range of ions of interest are created and stored in the ion trap during an ionization step. In one method, the r.f. voltage applied to the ring electrode of the quadrupole ion trap is then increased and trapped ions of consecutively increasing specific masses become unstable and exit the trap. These ions are detected to provide an output signal indicative of the masses of the stored ions.
In U.S. Pat. No. 4,736,101 there is disclosed a method of performing MS/MS in a quadrupole ion trap. In U.S. Pat. No. 3,527,939 there is described a method of isolating a single mass in a quadrupole ion trap. In this method, a combination of AC and DC fields are applied to the ion trap after an ionization step such that only the mass of interest will have stable or bounded trajectories and will remain trapped in the ion trap. All other masses, either above or below the mass of interest, will have unstable trajectories.
In U.S. Pat. No. 4,749,860 there is described a method of isolating ions of selected mass in quadrupole ion traps of the type including a ring electrode and end caps in which r.f. and AC voltages are applied to the ring electrode and end cap and scanned to trap a single ion of interest.
Ions stored within the quadrupole ion trap can be excited by applying an excitation voltage of predetermined frequency across the end caps of the ion trap. Ions that follow orbital trajectories at a frequency resonant with the excitation frequency gain kinetic energy as they absorb AC power. Two possible outcomes of this excitation are that the excited ions leave the confines of the ion trap and are no longer stored, or they undergo dissociation by ion molecule or ion/ion collisions within the trap (collision-induced dissociation).
The ability to assign an excitation frequency accurately so that the ions of a single mass-to-charge ratio may be excited is required for many of the operational modes of ion trap mass spectrometers. Rough excitation frequency assignments are possible by calculation of an ion's frequency of motion from the Mathieu equations for the ideal case of a single ion contained in a perfect quadrupole field. Relationships that account for all the variables affecting the frequency of an ion motion in an imperfect quadrupole field containing numerous ions have not been developed. In practice, the calculated frequency often differs significantly from those required to affect resonant excitation of an ion. Optimum resonant excitation frequencies can be experimentally determined by acquiring MS/MS spectra at a series of frequencies and plotting the results. The user determines the optimum resonant excitation frequency from these data by looking for the scan that shows the largest loss of parent ion signal and the maximum production of daughter ions. This procedure can take as long as fifteen minutes to perform, during which time constant sample conditions must be maintained. Automated programs that follow similar approaches have reduced the time required to obtain such data to a few minutes, but are still limited to continuous sample introduction methods.
For gas chromatography/mass spectrometry/mass spectrometry (GC/MS/MS), frequency assignments must be performed on a millisecond time scale. As the ion population within the trap changes over the chromatographic peak, the optimum resonant excitation frequency shifts.